1. Field of the Invention
The present invention relates to an image forming apparatus and a recording medium on which an interrupt control program is recorded.
2. Description of the Related Art
An electrophotographic image forming apparatus is capable of forming a high-quality image with favorable reproducibility and operability at low cost. This is why the electrophotographic image forming apparatus has been widely used in a copier, a printer, a facsimile machine, a multifunction machine having two or more of these functions just stated, and the like machine. The electrophotographic image forming apparatus includes, for example, a photoreceptor, a charging device, an exposing device, a developing device, a transfer device, and a fixing device. The photoreceptor is a member on whose surface an electrostatic latent image corresponding to image information is formed. The charging device serves to charge a surface of the photoreceptor. The exposing device serves to irradiate the charged surface of the photoreceptor with signal light to thereby form an electrostatic latent image. The developing device serves to supply a toner to the electrostatic latent image formed on the surface of the photoreceptor, to thereby form a toner image. The transfer device serves to transfer the toner image formed on the surface of the photoreceptor to a recording sheet. The fixing device serves to fix the toner image on the recording sheet to thereby form an image.
For the fixing device, for example, a fixing device including a fixing roller and a pressure roller is used. The fixing roller contains a heating portion therein which generates heat and fuses the toner constituting the toner image not yet fixed on the recording sheet to fix the toner image on the recording sheet in cooperation with the pressure roller. The pressure roller is disposed in pressure-contact with the fixing roller, and presses the recording sheet against the fixing roller when the toner is heated to be fused by the fixing roller, thereby promoting the fixing of the toner onto the recording sheet. A pressure-contact region between the fixing roller and the pressure roller is named a fixing nip region. The recording sheet bearing the not-yet-fixed toner image is introduced to the fixing nip region with a toner image bearing surface of the recording sheet in contact with the surface of the fixing roller.
Meanwhile, with a color image, an amount of toner used per unit area on the recording sheet is around two to three times larger than with a common monochrome image. Therein, in order to sufficiently fuse toner and obtain a good color fixed image, a color fixing device is adapted in such a way that the width of a fixing nip is increased by increasing the outer diameter of the fixing roller, increasing the thickness of a rubber layer formed on the fixing roller surface, or the like.
However, this kind of adaptation causes increasing the heat capacity of the fixing roller and, in the event that this kind of fixing roller with a high heat capacity is heated only by a heating source included therein, problems occur, such as a start-up time delay, or a decrease in fixing performance due to a decrease in temperature followability of the fixing roller at a continuous output time. Therein, in order to solve these problems, a configuration is proposed wherein a heating source is disposed outside the fixing roller, and the rate of temperature increase of the fixing roller is accelerated by heating the fixing roller from the surface thereof. With the fixing device with this kind of configuration, at a start-up operation time, by rotating the fixing roller, and heating the fixing roller from the exterior thereof using an external heating belt, the surface temperature of the fixing roller is caused to rise rapidly, achieving a reduction of start-up time.
With the configuration of heating the fixing roller from the exterior using an external heating device, unlike a configuration of heating the fixing roller from the interior using an internal heat source, the configuration is such that it is possible only to locally heat the surface of the fixing roller. Generally, it is necessary to heat while rotating the fixing roller in order to uniformly heat the fixing roller surface while transmitting heat by means of an external heating roller kept at a high temperature of around 200° C., a belt heat source, and a nip region formed by the fixing roller and external heat source. Specifically, when the image forming apparatus is powered on and started up, and when it is returned from a sleep mode, it is necessary, when uniformly heating the fixing roller surface using the external heating device, to rotate the fixing roller from a condition of a low temperature such as room temperature. The sleep mode is a mode in which a central processing unit (CPU) or the like is energized, but a heater of the fixing device is not energized, suppressing power consumption. During a standby period, by stopping the fixing roller, and uniformly heating the fixing roller from the interior thereof using a halogen lamp or the like provided inside the fixing roller, the temperature of the fixing roller is maintained at a certain temperature.
The temperature of the fixing roller immediately after the start-up is the highest on the surface thereof, and tends to decrease toward the interior (a metal core side) of the fixing roller, and contrarily, the temperature of the fixing roller in a standby condition is the highest inside the fixing roller, and tends to decrease toward the surface of the fixing roller. As the temperature distribution from the interior to surface of the fixing roller immediately after the start-up, and that in the standby condition, tend to be totally opposite to each other in this way, in the event of carrying out a continuous output immediately after the start-up, the fixing roller surface is rapidly deprived of heat by paper, making only a supply of heat from the external heating device insufficient to maintain the fixing temperature, and there is a fear that the fixability decreases.
Japanese Unexamined Patent Publication JP-A 2005-202359 discloses an image forming apparatus which, in order to ensure fixability, temporarily stops a job when the temperature of a fixing roller decreases to a reference temperature (hereinafter described also as a “job interrupt reference temperature”), and starts the job again when the temperature of the fixing roller recovers the reference temperature, thus ensuring the fixability.
However, with the image forming apparatus disclosed in JP-A 2005-202359, as the job interrupt reference temperature is uniformly determined without distinguishing between being immediately after the start-up and in the standby condition, there being a problem with a decrease in job efficiency and the fixability immediately after the start-up, a poor fixing or the decrease in job efficiency occurs when printing the job immediately after the start-up.